The present invention relates to a self-sustaining solenoid which moves a moving iron core by the application of an operating current and retains the moving iron core in its moved position even if the operating current is cut off.
Heretofore there has been proposed a self-sustaining solenoid of the type in which a moving iron core is moved into contact with a fixed receiver by the application of an operating current and a permanent magnet is used as the fixed receiver to thereby ensure that the moving iron core is retained in its operative position even if the operating current is cut off. With this conventional self-sustaining solenoid, also in its released state in which the moving iron core is held out of contact with the fixed receiver, the moving iron core is exposed to an attractive force by the permanent magnet forming the fixed receiver. Accordingly, there is the possibility that the moving iron core is moved by external vibration or shock even in the released state. If the distance between the moving iron core and the fixed receiver is selected large or if a strong return spring is provided for the moving iron core with a view to prevent such an erroneous operation, then the operating current must be increased to consume much power and the solenoid structure inevitably becomes bulky.
A solution to such problems is disclosed in U.S. Pat. No. 4,306,207 entitled "Self-Sustaining Solenoid", issued on Dec. 15, 1981. In the self-sustaining solenoid set forth in this patent, a moving iron core is divided into two in the direction of its movement and a permanent magnet is interposed therebetween and, as the permanent magnet, use is made of a magnet that is readily magnetized and demagnetized at room temperature. Applying an operating current to a coil of the self-sustaining solenoid, the moving iron core is moved by magnetic flux produced by the operating current into contact with a fixed receiver and, at the same time, the permanent magnet is magnetized by the magnetic flux, so that even if the operating current is cut off, the moving iron core is retained in its operative position by the permanent magnet. When to return the moving iron core to its initial position, a release current is applied to the coil and, by a magnetic field set up by the current, the permanent magnet is demagnetized, permitting the moving iron core to return to its original position by a small returning force. In addition, since the permanent magnet is demagnetized, it does not act to attract the moving iron core and, therefore, there is no fear of erroneous operation.
But the self-sustaining solenoid proposed in the abovesaid U.S. patent is complex in construction because of the provision of the permanent magnet in the moving iron core and has to be mechanically strong because the moving iron core repeatedly strike against the fixed receiver. Therefore, the split structure of the moving iron core is undesirable. Furthermore, as the permanent magnet is demagnetized in the released state, it is necessary that during operation the moving iron core be attracted only by the magnetic flux resulting from the application of the operating current. And when to return the moving iron core to its original position, the permanent magnet has to be demagnetized, so that the release current is also large, resulting in large power consumption.